How to rebuild the spotlight index on Papers 2 for Mac

The spotlight index files can become corrupted. A sign of this
is when library search is no longer working in Papers2, or in Magic
Citations. To solve such issues, you can rebuild Papers2 spotlight
index. This can be done very easily by starting Papers in
maintenance mode:

Quit Papers 2

Start Papers 2 again with the option key pressed (also called
the 'alt' key)

If you are rebuilding the spotlight index because Magic
Citations is unable to locate articles in your library, you may
want to restart Citations after rebuilding the spotlight index to
reload the application and restore functionality.